Halogenated solvents are used by a wide range of industries including dry cleaners, electronic equipment manufacturers, metal parts fabricators, insecticide and herbicide producers, and military equipment manufacturers. These solvents replaced petroleum derived mineral spirits and have distinct advantages because of their non-flammability. The persistence and mobility of these hydrocarbons in the subsurface was largely unanticipated, therefore historical disposal practices have led to widespread groundwater contamination. For example, trichloroethylene has been found at more than 791 of 1300 National Priority List sites, primarily as a groundwater contaminant.
Chlorinated solvents fall into the category of dense non-aqueous phase liquids (DNAPLs). DNAPLs are heavier than water and therefore sink below the groundwater table until they encounter a layer through which they cannot pass. As they move downwards, DNAPLs leave behind a smearing trace on their migration pathway before eventually pooling on a confining unit or perhaps within a crevice of a fractured rock. Most DNAPLs can dissolve in aqueous environments, yet they do so in such small quantities that the original contaminant pool functions as a subsurface contamination source. The portion of the contaminant that does dissolve is typically at concentrations which exceed allowable groundwater standards.
Treatment of halogenated hydrocarbon contaminated groundwater is usually accomplished by pumping the groundwater to the surface and removing the contaminant through oxidation or air stripping. Pump-and-treat remediation systems have experienced limited success with DNAPLs. Capillary pressure holds DNAPLs at residual saturation which can represent significant contamination. Consequently, removal of the contaminant from the subsurface is extremely time consuming, and cleanup goals are rarely achieved.
Because of the limited degree of success in remediating contaminated sites with technologies which attempt to remove the contaminant from the subsurface and pump it to a treatment system, recent efforts have focused on the physical, biological, or chemical treatment of these contaminants in situ. A permeable treatment wall is an alternative remediation technology which does not require groundwater to be pumped to a treatment facility. (Gillham, R. W., and Burris, D. R., "Recent Developments in Permeable In Situ Treatment Walls for Remediation of Contaminated Groundwater," Proceedings of Subsurface Restoration Conference, Jun. 21-24 (1992)). Instead contaminated groundwater is passively treated in situ. Permeable treatment walls, as shown in FIG. 1, are installed subsurface near a contaminant source. The process is passive in nature since natural groundwater flow transports the contaminants through the wall. Permeable treatment walls have been successfully demonstrated in several field studies and offer potential economic savings over other treatment methods.
Permeable treatment walls are designed so that larger volumes of water pass through the permeable treatment wall than through the surrounding soils. As contaminated groundwater flows through a treatment wall, halogenated solvents are chemically altered to give acceptable alternative species. Emerging on the downstream side of the treatment wall is contaminant-free groundwater. No pumps or other above-ground treatment are required, as the natural groundwater gradient carries the contaminant through the treatment wall.
Permeable treatment walls can be constructed using a mixture of a zero valent metal and a high permeability bulking material (e.g. sand or gravel). Alternatively, a permeable treatment wall comprising pure zero valent metal can be used. A number of techniques have been used to construct permeable treatment walls, including: 1) excavation and backfilling, 2) slurry trenching, and 3) borehole augering.
Traditional excavation and backfilling can be relatively cheap and expeditious if the depth of the excavation is shallow. However, with deeper depths, the shoring of the trench's side walls becomes a safety issue and can significantly slow down the progress of the excavation. Also, when excavating contaminated wastes, the costs associated with the ultimate disposal of the removed soil can be prohibitive. As a result, the excavation and backfilling method may not be the most economical construction method for large permeable treatment walls.
Slurry trenching is most commonly used to construct deep, impervious walls below the subsurface. Typically, the walls are made of concrete and are intended to contain a migrating plume of contaminated water, or to divert groundwater away from a contaminant source. During construction of slurry walls, a liquid mixture of water and bentonite (the slurry) is typically placed in an open trench to support the trench walls. After excavation, a cement slurry is pumped into the trench to form a permanent wall.
When applying traditional slurry trenching construction techniques to permeable treatment walls, bentonite can not be used, because the bentonite filter cake creates an impermeable barrier that defeats the objective of a permeable treatment wall. However, natural, biodegradable polymers can be substituted for bentonite. Typically, the bio-polymer maintains an effective filter cake for two weeks before dissolving in water. Once dissolved, the walls of the trench no longer prohibit water from passing through the treatment cell.
Thus, the use of slurry trenching to construct a permeable treatment wall eliminates the time consuming process of installing side braces, which is typically required for the traditional excavation and backfilling method. Unfortunately, the excavated soil disposal cost for slurry trenching is also high. For civil engineering applications, both trenching techniques usually do not extend to depths beyond 10 m.
Borehole augering is used throughout the drilling industry for the installation of pumping and monitoring wells. As adapted for permeable treatment wall construction, this construction method involves augering to a design depth, filling the borehole through the hollow stem auger with a coarse sand and zero valent metal mixture before removing the auger and leaving the new treatment column behind. The disadvantage of this construction technique once again stems from the excavation of contaminated soil waste. On average, for a 14-inch outer diameter borehole, approximately one 55 gallon drum of contaminated soil is generated for each five feet of augering. Thus, the costs associated with this construction method may also be prohibitive.
There is a current need for improved methods for constructing permeable treatment walls. In particular, there is a need for more cost effective construction methods that produce smaller volumes of excavated soil, and for construction methods that provide treatment walls with higher permeability.